The invention relates generally to x-ray tubes, and more particularly to structures and methods of assembly for the spiral groove bearing (SGB) utilized in an x-ray tube.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. In many configurations, the anode structure is supported by a liquid metal bearing structure, e.g., a spiral groove bearing (SGB) structure, formed with a support shaft disposed within a sleeve or shell to which the anode is attached and that rotates around the support shaft. The spiral groove bearing structure also includes spiral or helical grooves on various surfaces of the sleeve or shell that serve to take up the radial and axial forces acting on the sleeve as it rotates around the support shaft.
Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. The x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
Advantages of liquid metal bearings such as spiral groove bearings in x-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area. Other advantages include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal in the bearing structure, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube. However, liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the components that come into contact with the liquid metal bearing, such as the shaft of the anode assembly and is rotated for the purpose of distributing the heat generated at a focal spot.
As a result, the structure of the sleeve to which the anode is connected and the support shaft must be capable of withstanding the high temperatures and mechanical stresses created within the x-ray tube, as well as be able to withstand the corrosive effects of the liquid metal bearing. As such, a refractory metal such as molybdenum or tungsten is typically used as the base material for the construction of the sleeve or shell as well as for the other bearing components. Not only are such materials resistant to corrosion and high temperatures, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application. In addition, cooling of the bearing structure can be effected by flowing a cooling fluid into the center of the support shaft to thermally contact the heat taken from the anode by the sleeve and liquid metal bearing fluid.
Due to the low weldability of materials of this type to one another, in order to construct the sleeve around the support shaft to form the bearing structure, it is necessary to join the components to one another to form robust compression seals between the components of the sleeve capable of withstanding the operating pressures (≤1000 psi) of the liquid metal within the sleeve. These compression seals are formed by bolts that join the various component parts of the sleeve to one another. In order to prevent leaks from occurring along paths formed between the parts of the compression seals, anti-wetting coatings are applied to the surfaces within the compression seals to stop the flow of the liquid metal through the seals.
However, as the refractory materials are difficult to machine, these surfaces are hard to manufacture without surface imperfections that enable leaks to occur in the seals. Also, due to the low galling/wear properties of the refractory materials, these surface imperfections, even if not present after machining, can occur during normal use of the tube resulting in the formation of fluid leaks, thereby shortening the useful life of the tube.
In an alternative construction for a liquid metal/spiral groove bearing structure, other metals, such as steel, can be utilized in place of the refractory metals for the construction of the sleeve and support shaft, such as disclosed in U.S. Pat. No. 6,477,236. While these other metals have a lower resistance to corrosion by the liquid metal fluid, they have the benefits of low cost compared to the refractory metals, good machinability, good galling/wear characteristics, and good weldability. As such, these metals can be more easily constructed and joined to form the bearing sleeve.
However, as a result of the decreased resistance to corrosion from the liquid metal bearing fluid, it is necessary to employ complex thermal barriers in the construction of the bearing structure to limit the heat reaching the structure and causing corrosion of the structure by the liquid metal bearing fluid.
In one attempt to overcome the issues with these known x-ray tube constructions, U.S. Pat. No. 5,701,336 discloses an x-ray tube in which the component parts of a bearing sleeve or shell are indirectly joined to one another by soldering. In this construction, the various components of the sleeve are formed of a refractory metal, such as molybdenum, tungsten or an alloy thereof, and are positioned in an abutting position against one another with the adjoining surfaces of each component including an anti-wetting coating. The sleeve components are secured in that configuration by a number of adjacent connecting elements disposed on the exterior surfaces of the sleeve components around the abutting surfaces. The connecting elements are formed of a material that can be readily welded, such that the connecting elements can be welded to one another, thereby forming a joint over the abutting ends of the adjacent sleeve components.
However, in this construction, the use of the refractory metals, e.g., molybdenum, for the sleeve components retains the aforementioned issues concerning the leaks formed in the gaps between the sleeve components. In particular, deformations in the abutting surfaces and/or gaps in the anti-wetting coatings allow the liquid metal bearing fluid to pass between the components and react with the material forming the connecting elements, thereby forming leaks in the tube structure. The deformations would also occur or be formed as a result of alteration of the configuration of the molybdenum material resulting from the weld tempera an applied to the connecting elements.
Another alternative construction for an x-ray tube to address these issues is disclosed in U.S. Pat. No. 5,204,890, in which a thrust ring or bottom fixed disk is joined to a lower end of a fixed cylinder, with ceramic coatings applied to the surfaces of the disk and the cylinder that are designed to come into contact with the liquid metal lubricant. Due to the position of the ceramic coatings, the base materials used to form the disk and the cylinder forming the sleeve can be metal with a relatively low resistance to the liquid metal bearing fluid, such as an iron alloy, e.g., stainless steel or carbon steel. In one disclosed embodiment, the bottom disk and the cylinder are joined to one another by soldering to construct the stationary inner portion of the bearing structure.
However, in this construction, due to the structure of the thrust bearing/disk and cylinder adjacent the soldering/welding points, the process of joining the components to one another deforms the material of the disk and cylinder around the connection point, which alters the necessary tolerances between the components allowing leak pathways to form for the liquid metal bearing fluid, while also heating the liquid metal fluid to a temperature where it can corrode the material forming the cylinder, disk and connection point. In addition, the thrust bearing gap is controlled by the length of the sleeve which is harder to machine, the thrust bearing does not contain liquid when tilted significantly due to its geometry; and the bearing structure design is limited in thrust bearing capability as the journal diameter must be increased to create a large enough thrust surface, simultaneously increasing friction drag.
As a result, it is desirable to develop a structure and method for the formation of a bearing structure for an x-ray tube that can be formed with a simplified structure using low cost materials in a manner that significantly limits deformation of the materials to minimize the formation of leaks in the structure.